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Loans - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended
Jan. 31, 2010
Dec. 31, 2014
Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Residential mortgage loan   $ 20,800,000fmb_ResidentialMortgageLoan $ 24,300,000fmb_ResidentialMortgageLoan
Percentage of delinquent loan 2.85%fmb_PercentageOfDelinquentLoan    
Delinquent loans least percentage     0.25%fmb_DelinquentLoansLeastPercentage
Impaired loans classified as troubled debt restructured   797,200fmb_ImpairedLoansClassifiedAsTroubledDebtRestructured 861,200fmb_ImpairedLoansClassifiedAsTroubledDebtRestructured
Maximum time for re-evaluation (in months)   12 months  
Re-evaluation period for real estate   2 years  
Unsecured consumer loans, credit card credits and overdraft lines of credit reach   90 days  
Delinquent period for charging down consumer loans   120 days  
Delinquent period for charging down commercial and agricultural credits   120 days  
Consumer Real Estate [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans held for sale   $ 459,000us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fmb_ConsumerRealEstateMember		 $ 566,000us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fmb_ConsumerRealEstateMember
Maximum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Leverage position of balance sheet   1.50fmb_LeveragePositionOfBalanceSheet
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		  
Leverage position of customers   2.00fmb_LeverageRatio
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		  
Minimum [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Repayment period of loan with a reasonable reduction of principal balance   2 years